Large-screen, high-brightness electronic projection display apparatuses serve different broad areas of application such as e.g. electronic presentations for business, education, advertising, entertainment, simulation and status and information.
Electronic projection display apparatuses can have several designs. One of the optical designs often used, comprises mirrors that split an incoming light beam, which typically is a white light beam, in three different colour beams, typically a red, green and blue color beam, modulates the different colour beams with a light modulating means for each of the three different colours, and recombines the three modulated light beams in a dichroic prism, like e.g. an x-cube, to form a single light beam to be projected on a screen. Such a setup is e.g. described in patent application U.S. Pat. No. 5,934,778. The system has some specific advantages, as the light paths between the color splitting/recombining means and the light modulating means can be very small, as the light modulating means typically are positioned adjacent the X-cube dichroic prism, in slits attached to the X-cube dichroic prism. Furthermore, also in some alternative designs wherein the X-cube dichroic prism is only used for color splitting or for color recombination, i.e. whereby the light modulating means are of the transmissive form, it is advantageous to position the light modulating means for each color adjacent to the color splitting/recombining means, e.g. in slits attached to the X-cube dichroic prism. The group of a dichroic prism combination such as an X-cube, possible additional filters and the light modulating means with their holders, typically is called a convergence system.
For today's systems, there is a trend towards smaller light modulating panels having at least the same resolution or towards light modulating panels having higher resolution. Whereas in previous systems the typical dimension (e.g. length of side) of a pixel in a light modulating means panel was of the order of 30 to 40 micrometer, in current or future systems the typical size required is about 13 to 10 micrometer. If the same imaging quality needs to be obtained, much better fixed convergence systems, stable over time, and not drifting due to mechanical reasons, thermal reasons or humidity are required.
A typical demand is that the total shift during use of the system, i.e. after the system has been used for a significant amount of time so that it is influenced by environmental parameters such as temperature and humidity, is restricted to maximum of half a pixel. This requirement is based on the fact that the amount of color shift during use of the system should be reduced as much as possible. During production, this demand is translated into the requirement that the different components, e.g. the light modulating means panel and the X-cube dichroic prism, should be positioned relative to each other with an accuracy of the order of less than 1 micrometer.
Besides an optimized accuracy for positioning the different components of the convergence system by the initial assembly, also attempts need to be made to reduce the influence of environmental factors on the convergence system. This can be done either by decreasing the effects of the environment by e.g. sealing the convergence system from environmental factors, or reducing the variations in environmental factors for the convergence system, or by optimising the convergence system and its assembly so that it is less sensitive to variations in environmental factors.
U.S. Pat. No. 5,988,818 describes a projection type display for separating white light into three primary colors, forming images with liquid crystal light valves, mixing these images and projecting an enlarged picture of the mixed image. The system, which does not use an X-cube but uses color filters and different light modulating panels positioned separately on a chassis, provides several means to reduce the sensitivity of the system for environmental parameters such as heat. Means for adjusting the position of the different light modulating means panels to correct for misalignment and thus color and pixel shift are provided. Furthermore, the chassis plates and the different fixing means and holders are made of the same material such as steel so that the relevant parts have the same coefficient of thermal expansion. Therefore the parts of the adjustment mechanism uniquely expand and shrink. U.S. Pat. No. 5,988,818 also provides optimized cooling means to reduce the thermal variations of the environment and the components, and thus to reduce their effect on color and or pixel shifts. The document does not describe means to reduce the effects of environment on a convergence system using an X-cube.
OCLI, Optical Coating Laboratory Inc. describes on its website (www.ocli.com) an image kernel that uses an X-cube dichroic prism and provides a mechanical structure for mounting LCOS panels which is rigid such that convergence of pixels is said to be improved and thermally induced image variations are said to be decreased. Part of the thermal stability is obtained as the light beam is split up into color subbeams by a number of separate dichroic filters and wire grid polarizers which reduce the amount of heat generated in the X-cube dichroic prism.
Nevertheless, none of the above systems describes a convergence system having an image quality that is practically insensitive to environmental factors due to an optimum construction and assembly of the convergence system.